1. Field of the Invention
The invention relates to a control device for use in a resonant direct current/direct current converter.
2. Description of the Related Art
As shown in FIG. 1, a conventional LLC half bridge resonant direct current/direct current converter 8 performs voltage conversion or a direct current input power (not shown) so as to result in an output voltage (Vout′) adapted for driving a load (not shown). The conventional LLC half bridge resonant direct current/direct current converter 8 includes a switch unit 81, a resonating unit (not shown), a rectifying unit (not shown), and an output filter unit (not shown). The switch unit 81 is controlled by a frequency modulation controller 9, and affects magnitude of a current (IB′) (as shown in FIG. 2(I)) through the resonant inductor.
In particular, the switch unit 81 receives a first driving signal (drive1′) (as shown in FIG. 2(II)) and a second driving signal (drive2′) (as shown in FIG. 2(III)) from the frequency modulation controller 9, and receives the input direct current input power switchingly according to the first and second driving signals (drive1′, drive2′) for generating the output voltage (Vout′) through voltage conversion to be received by the load. The output power of the conventional LLC half bridge resonant direct current/direct current converter 8 varies with loading conditions, while the output voltage (Vout′) remains unchanged.
The frequency modulation controller 9 receives the output voltage (Vout′), and adjusts the frequency of each of the first and second driving signals (drive1′, drive2′) according to the output voltage (Vout′) adjuster of an output voltage adjuster, which is a part of the frequency modulation controller 9, so as to determine state of the switch unit 81 (i.e., to be switched on or off), to thereby achieve control of the output voltage (Vout′).
As the load becomes lighter, the output power is reduced. The frequency modulation controller 9 increases the frequency of each of the first and second driving signals (drive1′, drive2′). As a result, the switch unit 81, being controlled at a higher frequency by the first and second driving signals (drive1′, drive2′), switches at a higher rate, thereby increasing the number of switching actions. Consequently, the overall power conversion efficiency of the conventional LLC half bridge resonant direct current/direct current converter 8 is reduced.
U.S. Pat. No. 6,545,882 utilizes an off-time modulator that generates a discharge current to solve the above-identified problem.